The present invention relates to a track arrangement for carrying a measurement scale for use in scale, a scale therefor, and to apparatus and a method for tensioning a measurement scale.
A known form of opto-electronic scale reading apparatus for measuring relative displacement of two members comprises a length of scale on one of the members, having scale marks defining a periodic pattern, a read head provided on the other member, means for illuminating the length of scale, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the displacement.
An example of such apparatus is disclosed in EP-A-0 207 121 and also U.S. Pat. No. 4,974,962. Typically, the length of scale could be of spring steel with a surface layer of copper. U.S. Pat. No. 4,926,566 discloses a method of producing a length of scale, in the form of a flexible tape produced by rolling, the pitch of the scale marks being 20 xcexcm or 40 xcexcm for example.
Conventionally the length of scale may be carried by being received by a track, for example comprising an aluminium extrusion, the lower part of the track being attached to a surface, for example by screws or bolts, and the upper part of the track being formed to receive the length of scalexe2x80x94see FIG. 1 which is a section through a length of track 1 which is attached to a surface 2 by screws 3 spaced apart along its length and lengths of clamping strip 4, the upper part of the track 1 receiving a length of scale 5 which has been slid into it.
The upper part of the track 1 has scale holding elements 1a for keeping the scale in place.
However, such an arrangement can be difficult to make and to install. The small ride-height tolerance of a typical read head means that the track 1 requires accurate manufacture. The necessary clearance for the sliding fit of the scale 5 in the track 1 demands that the scale be manufactured even more accurately to compensate for this clearance.
It is not desirable to clamp the scale 5 securely to the track 1 because there may be relative thermally induced movement between the scale and track, particularly if dissimilar metals are used. For this reason the prior art track shown in FIG. 1 allows sliding movement of the scale. The scale will produce more accurate readings if it is held at a fixed position, both lengthwise and laterally but should not be clamped to the track, and should not be subjected to any deviations in the forces holding it in position.
According to one aspect of the present invention there is provided an elongate track comprising a channel, the channel being adapted for holding a measurement scale therein, wherein the track is adapted for attracting magnetically the scale to the channel of the track.
The track could have magnetic material (for example a length of flexible magnetic material such as ferrite rubber) for holding the scale magnetically. In this case, the length of magnetic material could be in the channel of the track, the scale being above the length of magnetic material, for example sitting on edge portions (which are, for example, below the upper surface of the track) along respective sides of the channel.
The track could be attached to the surface by screws passed through holes in it. Such holes could be at the bottom of the channel or in the track alongside such a channel.
The track could be machined or an extrusion, for example an aluminium extrusion.
Alternatively the track could be a length of magnetic material for example ferrite rubber, having the channel formed integrally therein.
The invention extends to a track and scale therefor.
It has been found that the scale may lift from a track or similar support under conditions where it is under compression. The present invention according to a second independent aspect relates to a scale tensioner, an embodiment of which prevents lifting of a scale.
One scale tensioning device is described in U.S. Pat. No. 4,559,707 (Heidenhain). However this and other scale tensioners are directed to correcting errors in the scale""s pitch rather than to prevent lifting.
The constructional details of these prior art tensioners differ from the proposed invention which provides a measurement scale tensioning device comprising a preloadable resilient member operable to urge the scale into tension.
The device may include a resilient member loading part operable to preload the member and operable to release the load.
The invention according to yet another aspect provides a method of tensioning a measurement scale, comprising the following steps:
providing a measurement scale, and scale tensioner;
preloading the scale tensioner;
mounting the scale to a substrate;
securing one end of the scale to the substrate and clamping the tensioner to the substrate adjacent the other end of the scale;
releasing the preload in the tensioner to cause a tension in the scale; and
securing the said other end of the scale to the substrate whilst maintaining the tension in the scale.
Preferably the method includes the step of providing a track to house the scale.
The invention extends to a method for tensioning and if necessary repeating the tensioning.